Pneumetic surface follower with position restoring force

ABSTRACT

Apparatus for measuring thickness, two light measuring heads (2, 3) being mounted by air bearings in housings on either side of the web (1). The measuring heads are provided with specially implemented nozzles blowing against the surface and maintaining a constant distance of 60-100 m to the respective paper surfaces. The thickness is measured by the distance between the nozzles being measured via an electrical measuring probe (13) in one, and a means (13&#39;) sensed by the measuring probe. By the special implementation of the nozzles (12, 12&#39;) and by their being pneumatically pressing (via 11) against the surfaces, great accuracy (better than 0.5 m) is achieved even if the position of the paper is altered, rapidity also being considerable. The time to indication for a thickness alternation of about 100μ is about 0.5 msec.

FIELD OF THE INVENTION

The invention relates to apparatus for measuring the thickness of amoving web, there being two axially opposite measuring heads, with theweb insertable between them, the measuring head being axially movable,supplied with a pressurized fluid and provided with ejection means forurging the pressurized fluid in a direction towards the web formaintaining a constant distance between the respective measuring headand the web, electrical transducer means being disposed in the heads forgenerating signals in response to the distance between the heads andthereby to the thickness of the web.

BACKGROUND OF THE INVENTION

Apparatus functioning according to such a principle is illustrated inU.S. Pat. No. 4,107,606. This publication only illustrates the saidprinciple very schematically, and is more specifically directed towardsa construction for electrical distance measurement between two measuringheads. How these are to be designed and implemented is not described indetail particularly as to the maintenance of distance providedpneumatically.

Another apparatus of the same kind is disclosed in U.S. Pat. No.3,528,002, although what is described in detail really consists of afixed measuring head, from which the web is caused to maintain aconstant distance while only the opposing head is movable. The distancemaintenance is described as taking place by compressed air being causedto emanate through a porous plug, for generating an air cushion and alifting force counteracted by a constant counterforce, which isillustrated as being generated by a relatively heavy mass. It is obviousthat this is a great disadvantage if a rapidly-acting thickness meter isdesired, since large mass results in large inertia.

Another prior art method for measuring thickness utilizes a singlemovable transducer, the position of the web being maintained fixedagainst an underlying plate, e.g., a suction plate, while the movabletransducer is kept by means of a pneumatic distance maintenance at aconstant distance from the other side of the web. Such a device is knownfrom U.S. Pat. No. 3,818,327, the distance between the suction plate andtransducer being measured electromagnetically. A similar device is knownfrom U.S. Pat. No. 3,617,872. With regard to the movable transducer, itis disclosed in both cases that an air cushion generated by compressedair is used together with a relatively heavy structure, thegravitational force of which counteracts the air pressure.

A substantially lighter structure for a transducer having pneumaticdistance maintenance is known from applicant's own Swedish PublishedSpecification No. 7900795-1. In this structure there is a light, movableand hollow measuring means, which is carried in air bearings, is axiallymovable in a holder, has at one end an air-blowing nozzle and at theother end a disc arranged with annular clearance in an outwardly opencylindrical chamber, the portion of the cylindrical chamber partiallyclosed off by the disc being supplied with compressed air such that acounterforce acts on the measuring means for urging it in a directionaway from the measured surface. This structure functions relativelywell, but it would be desirable to make it faster in assuming its stateof equilibrium, and to reduce the distance between nozzle and measuringsurface.

OBJECT OF THE INVENTION

One object of the present invention is to provide an apparatus formeasuring the thickness of a moving web of the kind described above, theapparatus having the ability of reacting very quickly, both measuringheads being able to follow the web, one on either side thereof, even ifthe web itself oscillates at right angles to its plane. It is thennecessary to have a large directional force for keeping the constantdistance between the measuring head and web on either side of the web,i.e., for an incident change in the distance between the web surface andthe measuring head, there must occur a large corrective force betweenthe web surface and the measuring head, and the measuring head must beextremely light. The directional force should thereby be large inrelation to the size of the moving mass.

The invention is based on the recognition that, apart from an "aircushion force" at an air nozzle, i.e., a force occurring in a spacebetween the web and the jet, there is also a relatively large "suction"force, such as occurs due to the Bernoulli effect, where the fluid exitsthe space between the nozzle and web in a laminar flow. This suctionforce should be smaller than the air cushion force, the difference beingcompensated with the aid of fluid pressure on the measuring head actingas a piston in an outward direction. Furthermore, the pneumatic forcesshould be adjusted such that the distance is maintained constant andthat a change in the distance results in a large readjusting force.

The above-mentioned and other objects and advantages are achieved inaccordance with the invention in that by implementing an apparatus ofthe kind discussed in the introduction such that the measuring headseach includes a housing with two radially acting air bearings, in whichis mounted a hollow stem, to the first end of which the respectiveejection means is attached; means for supplying pressurized fluid to theinterior of the stem; a closed first chamber which is pressurizable andin which the second opposite end of the stem is insertable; and in thatthe respective ejecting means comprises a space surrounded by an edgeand recessed in relation to the edge, the space obtaining pressurizedfluid from the pressurized fluid source via a constriction, and having across section at right angles to the axial direction having a largerarea than the cross-sectional area of the stem, the fluid pressure inthe closed first chamber acting in an outwardly urging direction.

A particular advantage of this arrangement is that the movable devicesassume their stable positions automatically upon pressurization.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail in conjunction withseveral embodiments shown in the accompanying drawings.

FIG. 1 illustrates an apparatus in accordance with the invention.

FIG. 2 is a section view illustrating a detail of a nozzle.

FIGS. 3A-3E are diagrams intended to explain the function of theinvention.

FIG. 4 illustrates pressure curves taken during measurement.

FIG. 5 is a dimensional sketch of a nozzle.

FIG. 6 illustrates an alternative embodiment.

FIG. 7 illustrates a safety device.

FIG. 8 illustrates an electrical connection.

DETAILED DESCRIPTION

An embodiment of the invention is depicted in FIG. 1, and has twomeasuring devices 2 and 3 arranged above and below, respectively, a web1 which may be moving, e.g., the paper web in a papermaking machine. Themeasuring devices may be movable in the direction transverse of the web,e.g., as illustrated in U.S. Pat. No. 3,528,002.

Since both measuring devices are rather similar, only the uppermeasuring device 2 will now be described. In a housing 4 there is arotationally symmetrical measuring head 5. This measuring head has astem 6 in the form of a thin aluminum tube, and a nozzle portion 12facing towards the web 1. The stem 6 is movable in two air bearings 7and 8, fed with compressed air via the housing 4. Between the bearings 7and 8 there is a chamber 9 which is also supplied with compressed air.In the portion of the stem 6 passing through the chamber 9 there are aplurality of holes, of which only the hole 10 is illustrated.

The end of the stem 6 facing away from the nozzle 12 thrusts into aclosed chamber 11 and is open. The nozzle 12 obtains compressed air fromthe interior of the stem 6.

In the nozzle 12 there is an electromagnetic sensor 13, for sensing thedistance to another sensing means 13' situated in a corresponding mannerin the substantially similar, lower unit 3. With the aid of flexiblewires 14 in the stem 6 the upper sensor 13 is connected via alead-through 15 to an electromagnetic measuring circuit.

In accordance with the functional principle of the invention, the nozzle12 is to be kept at a small, specific distance from the surface of theweb 1, and in such a manner that if the distance is changed, aconsiderable force will return the jet to the specified distance.

The design of the nozzle has great importance, and is shown in detail inFIG. 2.

As will be seen, the nozzle has the general form of a truncated cone 20,forming at the truncated end a slightly rounded annular surface 21,limited along its inner edge by the wall 22 of a recess with a bottomplane 23. In this plane is situated the electromagnetic sensor means 13.Where the latter penetrates towards the plane 23 there is formed anarrow annular gap with the width b. The depth of the recessed plane 23is denoted by a. The wall 22 forms an annular surface having a diameterφ₂, and the junction between truncation and cone 20 takes place along acircle having the radius /diam./φ₁.

In an embodiment which has functioned well, the outer diameter of thestem 6 was 7 mm, its inner diameter 5 mm, φ₁ was 10 mm, φ₂ was 9 mm, thedepth a was 0.2 mm and the gap b 0.20 mm.

It will be appreciated that the forces acting on the movable nozzleportion are gravity, a pneumatic force from compressed air, pressing thestem 6 outward of the chamber 11, the air cushion pressure in the spacebetween the web 1 and the forward end of the nozzle, and the suctionforce occurring when the air departs laterally in the gap between thesurfaces 21 and 20 and the web 1. The latter is, as is know fromBernoulli's law, proportional to the square of the air velocity in thegap.

The air supply was stabilized with the aid of a reduction valve of goodquality (e.g., Nordgren's precision regulator) so that a pressure of 1bar was supplied to the apparatus. The nozzles then adjusted themselvessuch as to be at a distance of about 80 μm from the surfaces.Sensitivity to variations in supplied pressure was insignificant. Adeviation in distance amounting to 3 μm has been measured for analternation of 0.1 bar, i.e., 10% of the pressure upwards or downwards.In view of the fact that a regulator set at 1 bar output pressure willmaintain the pressure within about 0.3% for an alteration of the inputpressure of between 5 and 7 bar, this source of error can be ignored.Well-reproducible measurement of the thickness could then be made atbetter than 0.5 μm, with the further advantage that within wide limits(±10 mm) the axial position of the paper does not significantly affectthe result. This is very advantageous in measurement during themanufacture of paper webs. It may be noted that the velocity of thepaper web only insignificantly and hardly measurably affects thicknessmeasurement, even for high speeds. The forces against the paper fromboth transducers tend to cancel out.

Previous tolerance requirements in the paper industry have been at ±5 μmin the manufacture of newspaper with an average thickness of 60 μm; thismay be greatly improved with the aid of the present invention.

It has been found that the apparatus according to FIG. 1 is ratherinsensitive to deviations from the vertical, and more or lessunresponsive to such deviations within 0°-45°, although the paper mustof course be trained at a corresponding angle. The reproducibility atturning the air on and off corresponds to 0.15 μm. (If the paper slopesby a couple of degrees in relation to a normal plane, no difference isexperienced in the measuring value, probably because the air bearingsprovide some compensation.)

The proximeter 13,13' used was of a standard type obtained from TSI(Transducer Systems Inc.), with the designation: Proximity transducertype XPTO15-022-250-750A.

The apparatus has been tried with success in a papermill in conjunctionwith a paper web having a velocity of 900 m/min.

The rapidity of answer is considerable. A "step" of 60 μm in the form ofa piece of adhesive tape on a paper 100 μm thick resulted in a 100%answer to the step in 6 msec, and it is presumed that a large part ofthe delay is due to the electronics. In a corresponding case in anexperiment with only one transducer, a rise time (10-90% of fullindication) of approximately 0.5 ms was measured.

It has been found that if such a step edge is successively inserted inthe gap between the two nozzles according to FIG. 1, full indication forthe thickness change is achieved when approximately 50% of the measuringsurface is covered by the thicker part of the web (the measuring surfacebeing reckoned as a circular surface with a diameter of 10 mm).

Consumption of compressed air is approximately 35-40 normal liters perminute, resulting in a cost for compressed air in full operation ofabout $300.00 per year. Prior art instruments working with single-sidedsensing consume about 10 times as much compressed air.

Although there is no complete theory available, the following discussionis an attempt to explain why the invention works so well.

In prior art instruments, it has been usual to build essentially on theair cushion principle. Air is ejected through several orifices into theintermediate space between a moving surface and a stationary one, themoving surface being loaded with a constant force. This force must beless than the applied pressure times the surface, as otherwise thesurfaces would come into mutual contact. For an increased spacing, theair cushion force decreases, and there is thus obtained in principle astable spacing with a somewhat smaller counterforce. Increased spacinggives less air-cushioning force, reduced spacing a larger air-cushioningforce, and a restoration force is obtained which favors restoration ofthe original state. The graph for force-distance is schematicallyillustrated by FIG. 3A.

A steeper curve may be obtained if the compressed air is supplied via asuitable constriction, since the maximum force (for direct contact) willremain unaltered, while the pressure is heavily reduced when airconsumption increases. This is schematically illustrated by FIG. 3B.However, there is also a suction force concerned with the presentinvention. This suction force is theoretically proportional to thesquare of the air velocity in the air gap. For a constant quantity ofair per time unit, the suction force will then increase rapidly when thedistance d decreases, and a suction force as illustrated in FIG. 3Ccould be obtained. Such a force curve is however such that no stabilitycan be achieved unless the counterforce, which is to balance it, isdirected for urging the nozzle away from the paper. A further difficultywith such an operational principle is that the air quantity per timeunit must be kept constant, and not the pressure. Maintaining a constantpressure at the nozzle is already difficult in view of constrictions inthe system. In more practical cases it is difficult to allow thepressure to increase for reduced distance, and therefore in the normalcase there is a force curve for the suction force which moves towardzero when the distance moves toward zero, and a curve having a change ofdirection according to FIG. 3D is obtained.

It is therefore presumed that the function of the invention correspondsto a combination of the curves in FIGS. 3B and 3D, both the suctionforce and air cushioning force being changed in the same direction for achange in the distance, and that the force counteracting the variableforces, and corresponding to the air pressure on the cross section ofthe stem 6, urges the nozzle towards the web.

FIG. 4 shows the measured static pressure under a measuring head forthree different applied pressures. The graphs do no appear to conflictwith the proposed explanation. They are recorded with a movable orificewith a diameter of 0.8 mm, made in a base plate and connected to apressure gauge.

The correctness of this reasoning and the importance of theconstrictions are supported by the fact that the size of the gap baccording to FIG. 2 has proven critical for stable operation. Forexample, if the gap b in FIG. 2 is made 0.35 mm, a stable if somewhatlarge distance is obtained only if the supplied pressure is heavilyincreased, rather good stability is obtained for the width 0.25 mm, andfor 0.20 mm there is obtained quite excellent operation withoutoscillations and with a moderate distance between nozzle and measuringsurface. The constriction is thus rather critical for the apparatus tofunction correctly and to give stable operation without oscillations.However, once those skilled in the art are aware of the importance ofthis factor, it is obvious that determination by testing does not entailparticularly great difficulties. An example of suitable dimensioning isotherwise apparent from FIG. 5.

In the foregoing explanation, the effect of gravity has been entirelyignored. This is justified since the mass of the moving nozzle is low.In the described embodiment, the moving mass in both units was only 4.5grams. This is to be compared with the forces caused by the airattaining several newtons.

In accordance with another embodiment illustrated in FIG. 6, and whichis rather like the units illustrated in FIG. 1, the orifices in the stem6 have been eliminated, and compressed air is supplied directly to theclosed chamber 11 via an opening 40. However, air supply to the airbearings can take place in the same way as previously shown.

A special safety device, illustrated in FIG. 7, is intended for theupper sensor unit, for ensuring against failure of the compressed air.It is obvious from FIG. 1 that if the compressed air to the lower unitfails, the dominant force will be gravity on the moving part, which willtherefore fall down. In the case of the upper part, however, the nozzlewould fall down onto the web, which would cause obvious problems if theweb is moving. To eliminate this, the structure in FIG. 5 has beenprovided with a single-acting, spring-loaded compressed air cylinder 50,fed from the same compressed air source as presses down the piston 51against the bias of the spring 52. Attached to the piston 51 there is asmall rod 53, the lower end of which is formed with a crosspiece. Therod 53 passes through a hole in a grid washer 54 rigidly attached to theupper end of the stem 6. If the pressure fails, the rod 53 will belifted and via the crosspiece at its end will lift the stem, therebypreventing the nozzle from falling down.

In the illustrated embodiments, the electrical connection to thetransducer 13 has only been illustrated schematically as twined-togetherthin wires 14. In practice, this is much too clumsy, and the arrangementschematically illustrated in FIG. 8 is used instead. The type oftransducer used according to the above requires three connecting wires.These are terminated in the upper part of the stem 6 in the chamber 11(FIG. 1) on the pillars 80. Inside the chamber there are the fixedconnection pillars 81 which are insulated and connected to the measuringequipment. Narrow phosphor bronze strips 82 connect the pillars 80 and81 and are arcuately curved to form three spirals. The force on themovable part will thus be minimal. Of course, the stem 6 must then notbe permitted to rotate, and to prevent this there has been provided inthe embodiment of FIG. 1 an axially extending pin inside the skirt 16accommodated with play in a recess in the edge of the "mushroom" head(not shown).

While compressed air has been mentioned throughout as the fluid ofchoice, it is obvious that any gas may be used, according tosuitability.

In the best mode embodiment known to date, it is important to note thatthe relationship between the surfaces is such that the effective aircushion surface is greater than the surface on which the air pressurepressing against the web is exerted, i.e., substantially the crosssection of the stem. The combination of the forces will thus be suchthat from experience there is obtained rapid and stabilized adjustmentto the web surfaces. Stability and rapidity are thus substantiallybetter than in applicant's previous invention disclosed in Swedish Pat.No. 7900795-1. The positioning distance will also be substantiallyshorter, in the order of magnitude of 60-100 μm on either side. Accuratethickness measurement is thus facilitated, since the distance measuredby the sensors will be smaller.

On theoretical grounds it is also obvious that, even if the restorationforce is large and the moving mass small, damping must be provided. Themushroom-like form of the nozzles together with the fixed skirting inthe embodiment according to FIG. 1 is quite sufficient for this purpose,and in the preferred embodiment the skirting has been provided withholes.

Unless particular means are provided for preventing turning, both themovable nozzle parts according to FIG. 1 may rotate freely about theiraxes. This is undesirable, at least with reference to the conductorwires 14, there being made an axially extending groove at the edge ofthe "mushroom", this groove accommodating with play a radial pin (notshown) arranged inside the skirting.

In the use of the apparatus, e.g., for measuring a moving web, it issuitable to take the apparatus outside the web at regular intervals andto allow the nozzles to blow against ech other. A given distance willthen be set up between them, which can be measured with the aid of thesensor 13 and its opposing part, and this distance should be the same oneach occasion for satisfactory operation. This distance does not,however, correspond to the case with an infinitely thin web 1, but doesgive good checking possibility even so.

I claim:
 1. A perpendicular surface follower for measuring the thicknessof a moving web, comprising two axially movable measuring heads arrangeddirectly opposite each other in an axial direction, for passage of saidweb (1) between them, a source of pressurized fluid for supplying saidmeasuring heads and ejection means for blowing said pressurized fluidtowards said web for maintaining a constant distance between arespective measuring head and said web, electrical transducer means (13,13') being arranged in said measuring heads for generating signalscorresponding to the distance between said measuring heads and therebyto the thickness of said web, each of said measuring heads comprising(a)a housing (4) with two radially acting air bearings (7, 8) bearing ahollow stem (6) having two ends; (b) respective ejection means (12, 12)being attached to a first end of said stem; (c) means (9, 10) forsupplying pressurized fluid to the interior of said stem; and (d) aclosed first chamber (11) which is pressurizable and which receives asecond end of said stem (6); and (e) the respective ejection meanscomprising a space recessed in relation to, and surrounded by, an edge(21), said space obtaining pressurized fluid from said pressurized fluidchamber via a constriction (b), said recessed space having a crosssection at right angles to said axial direction having a larger areathan the cross-sectional area of said stem, the fluid pressure in saidclosed first chamber (11) acting in an outwardly urging direction.
 2. Apneumatic surface follower for measuring the thickness of a moving web,according to claim 1, wherein said means for supplying pressurized fluidto the interior of the stem (6) comprise a second pressurizable chamber(9) traversed by said stem (6), in the wall of said stem having at leastone orifice (10) in its portion traversing said second chamber (9).
 3. Apneumatic surface follower for measuring the thickness of a moving web,according to claim 1, wherein said means for supplying pressurized fluidto the interior of said stem (6) comprise a pressurized fluid inlet (40)to said first chamber (11).
 4. A pneumatic surface follower formeasuring the thickness of a moving web, according to claim 1, whereinsaid constriction (b) is arranged as a narrow annular gap between acylindrical electrical transducer means (13, 13') and an orifice in saidrecessed space.